1. Field of the Invention
The present invention relates to an apparatus for controlling the moving speed of a magnetic head.
2. Description of the Related Art
In a conventional magnetic disk apparatus of, e.g., a sector servo type, position information is produced on the basis of servo data prerecorded on a disk as a recording medium, and positioning control of a magnetic disk is performed in accordance with the position information. This positioning control includes speed control and position control. In speed control, the actual moving speed value of the magnetic head is obtained in accordance with the produced position information, and the moving speed value of the magnetic head is adjusted such that the actual moving speed value coincides with a target speed value. That is, control is performed to position the magnetic head at a target position, i.e., a target cylinder (track). In contrast to this, in position control, when the magnetic head is moved to the target position by the speed control, the position of the magnetic head is adjusted such that the current position, of the magnetic head, which is based on the position information coincides with the target position. That is, control is performed to position the magnetic head at the center of the target cylinder.
A target speed value VR of the magnetic head is determined in accordance with a remaining distance XE from a current position Xn to a target position XR of the magnetic head. In accelerating the magnetic head, a maximum speed value Vmax is used. In decelerating the magnetic head, a speed value which changes as shown in FIG. 1 is generally used as a target speed value. Referring to FIG. 1, reference symbol V denotes the moving speed value of the magnetic head. In decelerating the magnetic head, the target speed value VR of the magnetic head corresponding to the remaining distance XE is calculated according to the following equation: ##EQU1## where S is a coefficient. However, it is difficult to perform the calculation of equation (1) by firmware. In practice, therefore, a speed table in which the target speed value VR corresponding to the remaining distance XE is preset is used to obtain a value similar to the target speed value VR calculated according to equation (1).
In this case, in order to obtain the accurate target speed value VR, a speed table having detailed position data shown in, e.g., FIG. 3, is required. That is, a speed table is required in which the remaining distances XE representing the distances between the current positions and target positions of the magnetic head are set as real numbers including zero and decimals such as 0.25, 0.50, . . . In contrast to this table, in a speed table shown in FIG. 4, the remaining distances XE are set as integers representing cylinder numbers, such as 0, 1, 2, . . . Therefore, by using the speed table shown in FIG. 3, a more accurate target speed VR can be obtained.
In order to use the speed table (FIG. 3) in which the remaining distances XE are set as real numbers, a large storage capacity is required as compared with the case wherein the speed table (FIG. 4) in which the remaining distances XE are set as integers is used. In addition, even if such a speed table is used, as the remaining distance XE becomes closer to 0, i.e., the current position Xn of the magnetic head becomes closer to the target position XR, an error in the target speed value VR is increased. For example, in the speed table shown in FIG. 4, if the remaining distance XE is a value between 249.75 and 250.00, the corresponding target speed value VR is 9.99 or 10.00. That is, the difference is 0.01. If, however, the remaining distance XE is a value between 0 and 0.25, the corresponding target speed value VR is 0.00 or 0.32. That is, the difference is increased to 0.32. This is because as the current position Xn of the magnetic head becomes closer to the target position XR, the speed curve of the magnetic head becomes steeper.
Furthermore, in the conventional magnetic disk apparatus having the speed table of FIG. 4, as shown in FIG. 5, as long as the magnetic head is located in the same cylinder (nth cylinder in this case), the current position of the magnetic head is indicated by the same value regardless of whether the magnetic dead is located at a position P1, P2, or P3 on the disk. Therefore, an accurate target speed value corresponding to the remaining distance cannot be obtained. As a result, as shown in FIG. 6, the moving speed value V to the target speed value VR greatly varies during deceleration of the magnetic head.
Note that a magnetic disk apparatus using a position detector such as a linear sensor is employed in order to accurately detect the current position of the magnetic head. Such a magnetic disk apparatus, however, requires a complicated arrangement, and an increase in cost is inevitable.
Since the moving speed value of the magnetic head greatly varies during deceleration of the magnetic head, the settling time is prolonged, resulting in an increase in the time required for seeking the magnetic head.
Under the circumstances, demands have arisen for a magnetic recording and reproducing apparatus which can accurately move the magnetic head to a target position at high speed.